A new way to increase GPS accuracy

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When the purposeful degradation of the civilian GPS signal (called selective availability or SA) was turned off on May 1, 2000, it seemed that the news on GPS accuracy couldn’t get and better: with SA off, a standard GPS receiver should be able to provide 12-meter (roughly 40-feet) accuracy anywhere in the world. And GPS accuracy can be improved still further by using the Coast Guard’s differential GPS (DGPS) radiobeacon network. Using DGPS corrections, most DGPS-equipped receivers can get 10-meter (30 feet) or better accuracy.

   Image Credit: Courtesy Raytheon

Now there's a second set of DGPS corrections available. Unlike the Coast Guard system, whose primary purpose is to serve mariners, these DGPS signals, called the Wide Area Augmentation System (WAAS), are provided by the Federal Aviation Authority (FAA) and are primarily intended for use by aircraft. However, anyone with a WAAS-capable receiver can use these corrections. And their method of delivery using geosynchronous satellite broadcasting on the GPS L1 frequency (1,575.42 MHz, more on that below) makes them readily available to users throughout the U.S. and for 200 miles offshore.

Like the Coast Guard's DGPS system, WAAS signals makes standard GPS more accurate; the FAA spec calls for seven meter (23 feet) vertical and horizontal accuracy - and WAAS tests have reportedly shown better accuracy than that. "We started out at five meters went down to three and now we're at one meter," said Craig Cushman, global marketing manager for Raytheon Marine. "Standard WAAS-equipped receivers are regularly showing sub-three meter (better than 9.84 feet) accuracy." However, as amazing as those accuracy numbers are, WAAS's greatest contribution to precise radionavigation for mariners may involve simplifying the process of getting DGPS corrections.

The Coast Guard DGPS system uses the existing set of marine radiobeacons as a broadcast platform. When the Coast Guard was developing its system in the late 1980s, this approach made sense. It was a relatively quick, inexpensive way of getting differential corrections on the air without building a whole new system. DGPS corrections are imposed on the radiobeacon signal using a modulation technique called medium shift keying (MSK). This MSK approach doesn't affect the radiobeacon signal and allows the corrections to be picked up by an appropriate receiver at the user's vessel. While this was a fast, efficient approach, it does have a few drawbacks. Radiobeacons transmit at the low end of the medium-frequency band (MF, around 300 kHz). Radio signals in this band are affected by radio frequency interference (RFI) caused by ionospheric effects at sunrise and sunset and thunderstorms (in the same way you hear the bursts of static on your car's AM radio when you see a lightning flash). And even if there is no thunderstorm in your area, if you are using a radiobeacon 50 miles away and a thunderstorm is between you and the beacon, the RFI can still affect DGPS signals. Other issues involve the lack of overlapping coverage in the radiobeacon network (if a beacon goes off the air, it can leave a hole in coverage) and the low data rate of the DGPS corrections. For mariners, who are used to operating in a fairly low-speed environment, none of these problems are a fatal flaw in the Coast Guard's DGPS system. For high-speed jet aircraft pilots who need precise en-route and landing guidance, though, the Coast Guard's DGPS setup is neither robust nor fast enough. Thus, the FAA developed its own DGPS system in WAAS. (There is a second FAA DGPS system called local area augmentation system or LAAS, but it is short-range and is designed to be used in the immediate area of major airports.)

Since aircraft must know their position, altitude, and speed very precisely in real time, a DGPS system for aircraft needs to provide fast updates, have overlapping coverage, and provide integrity (users have to know when the GPS-supplied position is in error). The basis for any differential GPS system is to compare a GPS position calculated on the fly with a monitor's precisely surveyed position. The difference between the two positions is the amount of correction required to remove errors from the GPS-derived positions. With SA turned off, the largest sources of error in a standard GPS-derived position are ionospheric delay of the signals, satellite clock drift, and the satellites' slight wavering as they fly in their orbits.

For detecting the errors in the signal, WAAS uses 25 ground-based monitor stations, called wide-area ground reference stations (WRS). These stations determine the errors and then send the data they have collected to the wide-area master station (WMS). The WMS correlates all the corrections and checks on the integrity of the system. A correction message is composed and relayed to a ground earth station (GES) where the message is uplinked to leased transponders on two Inmarsat geosynchronous communications satellites. From there, the corrections are broadcast on the same frequency as the GPS L1 signal, 1,575.42 MHz. A WAAS-capable receiver filters the WAAS correction message and only uses the corrections for the group of GPS satellites it currently has in view. When the corrections are applied, receiver ends up with a WAAS DGPS position that can be accurate to seven meters.

The overall WAAS architecture leads to certain advantages. For one, broadcasting on the L1 frequency removes the need to receive and process a sometimes noisy MF signal, thus eliminating an MF receiver chip set. This makes the receiver less complicated and eliminates the need for an MF antenna. Broadcasting the corrections using geosynchronous satellites allows much of North America and surrounding coastal waters to be covered by a single broadcast station, unlike the Coast Guard's radiobeacon network that requires many broadcast stations. And with two geo satellites available, there is a backup should one satellite go off the air. Furthering the value of WAAS is that fact that each geo satellite also broadcasts a GPS navigation message, effectively adding two more GPS satellites to the roster of satellites available to North American users.

To use the WAAS DGPS corrections, you need to have a WAAS-capable receiver; a marine radiobeacon DGPS receiver is not equipped to use WAAS. Since most modern receivers have multiple channels (eight to 12) to simultaneously track a number of satellites, assigning one channel to WAAS is not a problem. More important is having the appropriate software for decoding the corrections.

One manufacturer who has been very quick to incorporate WAAS capability its marine GPS receivers is Raytheon. As the FAA's prime contractor to develop WAAS, Raytheon has a natural understanding of the technology and ready access. "Having connections with the guys in the WAAS department [of Raytheon] doing the work gave us the jump," said Raytheon's Cushman. In the early summer Raytheon introduced the WAAS-capable RayNav 300 and Raychart 320 receivers.

Raytheon isn't the only forward-looking GPS company. The other GPS manufacturer now offering WAAS units is Northstar. "It's just a matter of keeping our eye on the ball," said Bruce Angus, vice president of sales and marketing for Northstar Technologies. "We see this as a coming thing." Northstar Technologies offers the 952XW. Northstar plans to continue to offer the radiobeacon DGPS version of the 952, the 952XD. Other GPS manufacturers will, no doubt, follow suit with WAAS-ready machines.

Right now, WAAS coverage is only slated to be available in North America. However, a European system called the European geostationary overlay service (EGNOS) is reportedly on the way. This setup will provide a WAAS-like signal for GPS users in Europe. The Japanese are working on a system called MSAS that provides a similar service in Japan. Both of these systems should be fully WAAS compatible.

By Ocean Navigator